Heretofore, a known hydraulic pump control system for controlling the delivery rate of a variable displacement hydraulic pump has been proposed in, e.g., JP-U-5-64506. This conventional control system comprises a variable displacement hydraulic pump, a hydraulic actuator driven by a hydraulic fluid delivered from the hydraulic pump, a flow control valve of a center bypass type controlling the flow rate of the hydraulic fluid supplied to the hydraulic actuator, a center bypass line extended from the hydraulic pump to a reservoir while penetrating a center bypass of the flow control valve, a throttle valve disposed in the center bypass line on the downstream side thereof for generating a control pressure (negative control pressure), a sensor for detecting a value of the generated control pressure, a controller for receiving a signal from the sensor, calculating a target pump tilting of the hydraulic pump and outputting a drive current corresponding to the calculated target pump tilting, a proportional solenoid valve for producing a command pressure corresponding to the drive current, and a regulator for controlling the tilting of the hydraulic pump in accordance with the command pressure from the proportional solenoid valve.
In the above construction, when the stroke amount of the flow control valve is small and the flow rate of the hydraulic fluid passing through the center bypass line is large, i.e., when the pressure upstream of the throttle valve is high, the controller calculates the target pump tilting so as to reduce the delivery rate of the hydraulic pump. When the stroke amount of the flow control valve is large and the flow rate of the hydraulic fluid passing through the center bypass line is small, i.e., when the pressure upstream of the throttle valve is low, the controller calculates the target pump tilting so as to increase the delivery rate of the hydraulic pump.
Further, the controller converts the target pump tilting calculated as mentioned above into a command value in the form of a drive current, and then outputs the drive current to the proportional solenoid valve. As a result, the proportional solenoid valve outputs a command pressure corresponding to the drive current, and the hydraulic fluid is delivered from the hydraulic pump at a flow rate corresponding to the target pump tilting.